Hitherto, optical discs are widely used as an optical recording medium. As the optical disc, there are used an optical disc of the reproduction only type in which information signals have been recorded in advance by micro uneven patterns, and/or an optical disc in which phase change material or magneto-optical material is used for recording layer to permit recording of information signals.
Among them, in an optical recording medium drive apparatus such as optical disc apparatus, etc. using, as recording medium, an optical disc in which recording and reproduction of information can be made, semiconductor laser having relatively large maximum emitting light quantity (optical output maximum rating) has been generally used as light source of the optical head. It is to be noted that light source having large maximum rating is not required in the reproduction (playback) only apparatus. The reason thereof is as follows.
Namely, in the semiconductor laser, generally speaking, in the case where output is small, it is difficult to obtain stable oscillation so that laser noise becomes large. Accordingly, in order to ensure C/N ratio (Carrier to Noise Ratio) at the time of reproduction of information, it is necessary to set light output of laser to a value more than a predetermined value. This value is ordinarily about 2 mW to 5 mW.
Moreover, in optical recording media which permit recording of information, recording is performed by using heat elevation, etc. of recording layer resulting from the fact that light beams are converged onto the recording surface of the optical recording medium. In this case, when attempt is made to satisfy two conditions where recording signal is not deteriorated in the reproduction light power, and stable recording is performed in the recording light power, it is necessary to ensure a predetermined output ratio or more in the light power at the time of reproduction and in the light power at the time of recording. Ordinarily, the maximum power of recording light is greater about 5 to 20 times than the reproduction light power. Further, in such cases that recording is performed at a speed higher than the standard speed, larger output ratio is required.
For these two reasons, the light output maximum rating of light source used for optical head which copes with recording and reproduction, and/or light source used for optical head which performs recording and/or reproduction with respect to plural types of optical recording media is ordinarily 20 mw to 50 mW, and becomes so far as about 100 mW in the optical recording medium which performs recording operation at a high speed which is greater about eight times than the standard speed, e.g., optical disc of the so-called “CD-R/RW” system, etc.
The light source having large light output maximum rating as described above has the problems that not only its realization is very difficult, but also power consumption in the light source becomes large, etc. In addition, when attempt is made to use such light source in the state where light output is small as in the case of time period of reproduction, laser noise becomes large so that satisfactory reproduction characteristic cannot be obtained.
On the other hand, in DVD-ROM disc which is reproduction only DVD (Digital Versatile Disc), etc., optical discs having two recording layers have been already put into practice. Also in regard to optical discs which can perform recording and reproducing operations, optical discs having multi-recording layers such as two layers or four layers are being proposed.
In order to perform recording/reproduction onto these multi-layer optical recording media, recording light power and reproduction light power which are greater about 1.5 to 2 times or more than that of the optical recording medium having only one recording layer have been required. For this reason, in the case where a system which can use optical recording media having single recording layer and multi-recording layers is assumed, ratio between the recording maximum light power when recording is performed onto multi-layer optical recording medium and reproduction light power when reproduction of optical recording medium having single layer is performed approximately becomes double as compared to the case where only the optical recording medium having single recording layer is handled.
Further, when linear velocity of the optical recording medium is changed, recording light power and reproduction light power necessary for light beams which scan the optical recording medium are changed. Namely, when linear velocity of the optical recording medium with respect to light beams becomes large, larger recording and reproduction light powers are required.
As stated above, it is required that the dynamic range of light output of light source should be broader in accordance with future expansion of the recording capacity.